SALT LAKE CITY — Utah health officials on Tuesday said they have been unable to determine how the son of an elderly Salt Lake County man sickened by the Zika virus also contracted the virus.

The mysterious case baffled public health officials, who said the son did not have any of the known risk factors for Zika, such as travel to an affected area or sexual contact with an infected person.

The son had been helping take care of his father, who died due to Zika-related complications in June, according to officials.

Utah Department of Health spokesman Tom Hudachko said health officials tested more than 200 people for the virus, including family contacts, health care workers and neighbors. No additional cases of Zika virus have been identified as part of the investigation.

Increased mosquito abatement efforts have not found any of the two mosquito species that transmit Zika, the health department said.

“An important part of any public health investigation is working to identify additional cases of disease,” said Utah Department of Health deputy state epidemiologist Dr. Angela Dunn. “Finding new cases can help lead us to answers during an investigation, and while we’re happy nobody else was infected, the lack of additional cases leaves many questions unanswered.”

The U.S. Centers for Disease Control assisted with the Utah investigation and its preliminary findings are detailed in a report that was released on Tuesday along with the CDC's weekly morbidity and mortality report.

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Nearly 100 people in the Salt Lake County neighborhood where an elderly man died after contracting Zika virus have submitted blood samples for testing, according to public health officials.

In the report, researchers said the elderly Salt Lake County man developed septic shock with multiple organ failure and died in the hospital on June 25.

They noted that the man was infected with a uniquely high amount of virus — about 100,000 times higher than average levels seen in other samples of infected people — in his blood.

"It remains unclear how (the son) was infected," the report states. "However, (he) was known to have had close contact (i.e. kissing and hugging) with the (father) while the (father's) viral load was found to be very high. Although it is not certain that these types of close contact were the source of transmission, family contacts should be aware that blood and body fluids of severely ill patients might be infectious."

The report added that when there are high levels of virus in a patient's blood, "it is essential that health care workers continue to apply standard precautions while caring for all patients, including those who might have Zika virus disease."

Zika virus is transmitted to people primarily through the bite of an infected mosquito. Zika can also be passed through sex and from mother to fetus during pregnancy.

Researchers are investigating other modes of transmission. Some believe the mosquito-borne virus could theoretically be transmitted through blood because of evidence in some labs of exposure when samples were not handled correctly.

Utah health officials said the investigation will remain active.

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